Remove need for attached test-jar artifacts

Remove test-jar artifacts from Maven projects and relocate classes. The
majority of utilities now live in the `spring-boot-testsupport` module.

This update will help us to deploy artifacts using the standard Maven
deploy plugin in the future (which doesn't support the filtering of
individual artifacts).

Fixes gh-9493

spring-boot-tools/spring-boot-loader/pom.xml

@@ -47,6 +47,11 @@
 			<version>1.46</version>
 			<scope>test</scope>
 		</dependency>
+		<!-- Test -->
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-test-support</artifactId>
+		</dependency>
 	</dependencies>
 	<profiles>
 		<profile>